The long term viability and propagation of all living cells depends upon DNA repair enzymes that process abasic sites created by the loss of the associated purine or pyrimidine base. A key enzyme for the detection and processing of these DNA lesions is Endonuclease IV, an evolutionarily conserved ~30 KDa Zn-dependent metallo-endonuclease. We have grown crystals of Endonuclease IV from the mesophilic and hyperthermophilic bacteria E. coli and T. maritima, respectively, and both crystals diffract x-rays to greater than 1 E as evidenced by preliminary data sets. Ultrahigh resolution diffraction experiments on BL 9-1, using long exposures and an optimized wavelength, will yield sub-Engstrvm data that will be used to define the structural biochemistry of this critical DNA repair enzyme.